Left 4 Dead 2

Now that the game is actually working fully it would seem, I was wondering if anyone else has experienced this subtle bug. *Only* special infected and tanks seem to be going invisible. I have most/all my graphical settings set to low, so this could be why. If anyone knows of the exact graphical setting to change in order to prevent this, this would be helpful(preferably I would like to turn my graphics up as minimally as possible because the game is FINALLY running relatively smooth.)